Organomodified siloxanes, especially polyether siloxanes, are extensively used compounds in almost all sectors concerned with directed control of surface-active behavior. Substances of this type are used for example as surfactants, emulsifiers, dispersants, paint flow additives, lubricants, defoamers, as auxiliaries in enhanced oil recovery, as textile auxiliary for coating fibres, yarns or fabrics or as foam stabilizers in polyurethane foaming. The sheer variety of uses for this class of chemicals rests not least on the possibility of using a suitable combination of siloxane structures and polyethers as substituents to specifically obtain, alongside the hydrophilic/hydrophobic balance, other types of activity.
Polysiloxane-polyoxyalkylene copolymers, which contain modified units of polyoxyalkylene, are deserving of particular respect for use as an additive in the production of polyurethane foams. These foam stabilizers enable for example the formation of a uniform pore structure and stabilize the polymer matrix during foam production. The effectiveness of a polysiloxane-polyoxyalkylene copolymer in foam stabilization can only be predicted to a limited extent.
Polyether-polysiloxane copolymers are basically subdividable into two classes. Polyether-polysiloxane copolymers where the polyethers are linked to the polysiloxane chain via an SiOC bond are long known. Such copolymers are prepared by reacting hydroxyl-functional polyethers either with chlorosiloxanes in a substitution reaction or with alkoxysiloxanes in a transesterification reaction. This group of products is particularly notable for a wide processing latitude and a high activity, i.e., polyurethane foams of fine cell structure, the desired open/closed cell content and free of foam defects are obtained over a wide concentration range of the stabilizer. Polyurethane (PU) stabilizers of the SiOC product group are by virtue of these properties widely useable in a multiplicity of formulations. One disadvantage is the limited hydrolysis resistance of the SiOC bond, which limits the shelf life of the stabilizers and more particularly of their admixtures or formulations. A further disadvantage of SiOC stabilizers is their moderate solubility in polyol formulations, which can cause clouding or even signs of separation of the fully formulated polyol recipe. This issue presents particularly with rigid foam applications where preformulated mixtures containing polyols, catalysts, water/blowing agent, the foam stabilizer and optionally further additives, which are marketed as ready-to-use polyurethane foam systems, are frequently required to have solubility and separation resistance.
A desire to address the issue of poor stability in storage motivated the development of a second class of polyether-polysiloxane copolymers where the polyethers are attached to the polysiloxane by hydrolysis-stable SiC bonds. Preparation is by noble metal-catalyzed hydrosilylation of allyl polyethers with hydrogensiloxanes. PU stabilizers of the group of SiC products are notable for high solubility in polyol formulations as well as their good stability in storage. However, their use in hot-cure flexible foam applications does not enjoy the benefit of the broad processing latitude known from SiOC stabilizers, since the amount of stabilizer used has to be kept constant within a narrow range in order that consistently good foam properties may be obtained.
Platinum metal-catalyzed hydrosilylation is the currently practiced standard reaction for hydrolysis-stable organomodification of hydrogen siloxanes. The industrial synthesis of polyether siloxanes linked Si—C largely rests on using the easily accessible allyl/vinyl polyethers.
The polyoxyalkylene block can be altered with regard to its composition of oxyalkylene units, especially oxyethylene, oxypropylene and oxybutylene. The term composition comprehends not only the relative number of oxyalkylene units represented therein, but also their distribution/arrangement in the polyether chains. Further characteristics are the molecular weight and the end group of the polyoxyalkylene block.
End groups customary in polyoxyalkylenes incorporated in commercial flexible polyurethane foam stabilizers are essentially the hydroxyl functionality, the methyl ether group, the butyl ether group or else the acetate function.
It has now been the case for some years that polyurethane foam systems are gaining ground which utilize high-pressure carbon dioxide as a blowing gas. This technology is described in EP-A-0 645 226 for example. The characteristic of this foaming technology is that the spontaneous frothing of the pressurized CO2 as the PU reaction mixture is discharged places increased requirements on the cell formation characteristics of the components in the foam formulation. This is readily understood since conventional PU foaming is characterized by the slowly starting isocyanate-water reaction, which only produces slow CO2 gas saturation of the liquid polymeric phase and subsequently a hesitant formation of gas bubbles.
Systems of this type are a technical challenge in that, despite the carbon dioxide turning into a gas within a fraction of a second, it is necessary to control the cell count and the cell size distribution as parameters of the later foam morphology and the foam properties resulting therefrom. Typical flaws due to inadequate process control are nonuniform, partly coarsened cells within the foam structure.
Ways to minimize these flaw scenarios reside, for example, in selecting suitable polyether-polysiloxane copolymers as foam stabilizers, as described in U.S. Pat. Nos. 5,357,018 or 5,321,051 for example, or in adding cyclic organic carbonates in polyurethane foam production especially in flexible polyurethane-polyether foam formulations, as taught in EP 0 900 811.
EP 0 798 327 describes a process for the preparation of polycarbonate copolyether diols in the form of a two-stage process. In a first step, a polyether glycol is reacted with bisdimethylcarbonate in the presence of a basic catalyst to form a polyether diol bisdimethylcarbonate which, in a subsequent second step, after distillative removal of the excess dimethyl carbonate and removal of the basic catalyst, is converted by acid-catalyzed transesterification into a polycarbonate-co-polyether diol. A disadvantages of such a preparation method resides in the working up of the intermediate step and the need to change the catalyst system.
U.S. Pat. No. 5,525,640 discloses silicone-based wetting agents and their use in polyurethane foam blown with inert gases. The core concept taught by this reference is that a polyurethane foam under inert gas pressure must ideally be stabilized using a comb-structured polyether siloxane where the ethylene oxide content of the polyether accounts for less than 37% of total alkylene oxide content. The teaching is supported by comparative foaming tests in mechanically wiped but also liquid CO2 blown foam system on completely acetoxy- or methyl-capped comb-type polyether siloxanes. Alternative end groups disclosed for capping the polyoxyalkylene branches are —C(O)Z′, —C(O)OZ″ or —C(O)NHZ′ where Z′ comprises monofunctional alkyl or aryl groups of 1 to 8 carbon atoms. No particular importance is attributed to these end groups, as is also evident from the fact that acetate endcapping and methyl endcapping are preferred.
WO 03/091320 concerns silicone-based wetting agents particularly useful for rendering CO2-blown polyurethane foam flame retardant when the cell structure is fine. The disclosure of U.S. Pat. No. 5,525,640 is taken up once more in again claiming comb-type polyether siloxanes where the ethylene oxide content of the polyether accounts for less than 37% of total alkylene oxide content and also, in addition to the specifically claimed endcappings with acetate and methyl groups also, in the polyoxyalkylene moiety, —C(O)Z′, —CC(O)OZ′ or —C(O)NHZ′ where Z1 comprises monofunctional alkyl or aryl groups of 1 to 8 carbon atoms.
Similarly, U.S. Patent Publication No. 2010/0286295, which concerns silicone-based wetting agents for use in polyurethane foams derived from vegetable oil polyetherols and which claims the special structural principle of alkyl-containing silicone polyethers, again describes the possibility of organocarbonate endcapping disclosed in WO 03/091320. Again, acetate or methyl endcapping is preferred.
Notwithstanding the progress made to date, available systems still lack any distinct improvement in the cell structure of polyurethanes in critical formulations or in foaming with liquid, pressurized gases.